I didn't make the lamp, but discussed the process with the artisan that did (https://www.ragsdalehomefurnishings.com/). I recall they were just 45 degree miters. I've had them over 20 years. At the time he was still making them in a garage shop.
I understand a threaded tube runs through the center as a wireway and hardware anchor. I have a couple of his Table Lamps too. They are constructed similarly. Beautiful work.